The differences in mechanisms of nitrogentransfer between plasma(ion) nitriding and gas nitriding have variouspractical consequences.For example, gas nitridingincreases the surfaceroughness of gray castiron. Proper identificationof the advantages and disadvantages of thesetwo methods could significantly improve their use.

Edward Rolin’ski**Advanced Heat Treat Corp. Monroe, Mich., and Waterloo, Iowa

A. Konieczny*U.S. Steel Automotive CenterTroy, Mich.

Gary Sharp**Advanced Heat Treat Corp. Monroe, Mich., and Waterloo, Iowa

**Member of ASM International and member,ASM Heat Treating Society*Member ASM International

The mechanism of plasma (ion)nitriding has been studied ex-tensively during the past 40years[1-12]. There is general

agreement that nitrogen ions andradicals such as N+, N2+, NH+, NH2+,as well as fast neutral nitrogen mol-ecules play a significant role in ni-trogen transfer. This occurs either bygenerating active nitrogen atoms onthe cathodic part surface[6], or by at-taching to iron atoms sputtered fromthe surface and redeposited back onthe cathode in the form of nitrides[4].The gas-nitriding mechanism usingammonia is less controversial andhas been considered as a catalytic re-action of ammonia with iron whichleads to the formation of the active

nitrogen atoms in statu nascendi andtheir chemisorption according to thereaction[13, 14]:

2NH3 —> 2Nad+3H2

The differences in nitrogen-transfermechanisms between the twomethods have various practical con-sequences. For example, plasma ni-triding is considered as a low ni-triding-potential method, unable toproduce a thick compound zone[13],and plasma nitriding does not pene-trate small holes, cavities, etc., and,therefore, the process can be used forhardening sintered low-density ironparts[15, 16]. This characteristic ofplasma is also used to advantage in

INFLUENCE OF

NITRIDING MECHANISMSON SURFACE ROUGHNESS OF PLASMA AND GAS NITRIDED/NITROCARBURIZED GRAY CAST IRON

View of a large stamping die undergoing plasma nitriding in a vacuum chamber.

HEAT TREATING PROGRESS • MARCH/APRIL 2007 01

mechanical masking. By comparison,gas nitriding can produce a com-pound zone (white layer) of practi-cally any thickness by controlling thenitriding potential using hydrogenor oxygen sensors and modern auto-matic methods[19-21]. Also, gas ni-triding can be used for hardeningparts having small holes, such asthose in diesel injector plungers, andnitriding the entire surface of anyproduct. Proper identification of therespective advantages and disadvan-tages of the two methods could sig-nificantly improve their use.

The differences in plasma and gasnitriding are demonstrated oncommon gray cast irons, which arefrequently used for tooling in manystamping operations[22]. This inex-pensive material offers a good com-

bination of cast mechanical proper-ties and damping, which are very im-portant in large, 20-30,000-lb (9-13,600 kg) tooling. High-quality finishing of such tools to Class A-surface requirements after thermo-chemical treatment is of paramountimportance to achieve good, longlasting performance in manufac-turing applications. Surface qualityrequirements are even more criticalfor tools designated for stampingmodern materials such as advancedhigh strength steels (AHSS), in-cluding DP490 dual-phase steel.Therefore, post-nitriding roughnessmust be kept to a minimum and anyfinishing operations should not be toocostly. The authors observed that graycast iron dies become rougher aftergas nitriding and require more fin-

Fig 1 — Surface of gray cast iron samplesafter gas nitrocarburizing at 1050ºF, or 566°C(a and b), gas nitriding at 968ºF, or 520°C (cand d), and plasma nitrocarburizing at1050ºF (e and f).

a

b

c

d

e

f

Fig 2 — Profilograms of samples after gas nitrocarburizing at 1050ºF, or 566ºC (a), gasnitriding at 968ºF, or 520°C (b), and plasma nitrocarburizing at 1050ºF, or 566°C (c).

Ra 126.99 microin. (3.25 µµm)Rmax 951.57 microin. (24.17 µµm)

P Profile Leveled Lc/Ls = Off2,500.0

0.0

-2,500.0

1,000.0

0.0

-1,000.0

500.0

0.0

-500.0

Pick-up TK300 Lt = 0.200 in. Vt = 0.020 in./s 0.200

Mic

roin

.M

icro

in.

Mic

roin

.

Ra 121.67 microin. (3.09 µµm)Rmax 989.84 microin. (25.14 µµm)

P Profile Leveled Lc/Ls = Off

Pick-up TK300 Lt = 0.200 in. Vt = 0.020 in./s 0.200

Pick-up TK300 Lt = 0.200 in. Vt = 0.020 in./s 0.200

Ra 35.67 microin. (0.91 µµm)Rmax 322.87 microin. (8.20 µµm)

P Profile Leveled Lc/Ls = Off

02 HEAT TREATING PROGRESS • MARCH/APRIL 2007

ishing than the plasma-nitrided dies,which has a high possibility of beinga result of differences in the nitridingmechanisms of the two methods.

ExperimentalAll experiments were performed

on G3500[22] gray cast iron sampleshaving the following chemical com-position in wt%:

2.8-3.2C, 1.5-2.2 Si, 0.7-1.0 Mn, 0.35-0.50 Cr, 0.35-0.50 Mo, <0.7 Cu,

<0.15 S, <0.15 P, bal. Fe

Samples were ground using metal-lographic polishing papers, finishingwith 600× grit. Plasma nitrocarbur-izing was performed in a nitrogenand hydrogen gas mixture con-taining 2% methane. Controllable gasnitriding was performed in an am-monia, nitrogen, and hydrogen gasmixture, and also with carbondioxide in the case of the nitrocarbur-izing cycle. The gas nitriding systemwas equipped with a hydrogensensor and automatic control of thenitriding potential. Process parame-ters are shown in Table 1.

ResultsHigh-temperature (1050ºF or 566°C)

gas nitrocarburizing caused a signif-icantly rougher surface. Steps takento minimize the problem includedtrying nitriding instead of nitrocar-burizing and using a lower pro-cessing temperature (968ºF, or 520°C) and lower nitriding potential. Theseprocessing conditions reduced sur-face roughness compared with thenitrocarburized samples, but surfaceroughness was still unsatisfactory.By comparison, plasma processingresulted in a significantly lower sur-

face roughness with very few openmicrogrooves and cavities in the sur-face (Figs. 1 and 2). Data on surfaceroughness after processing are shown in Table 2. Rmax for theplasma-nitrided surfaces, althoughmuch lower than after gas nitriding,are still comparatively high forstamping tool applications. However,it should be noted that these picks aremade of a loose deposit of the nitridesfrom plasma, which do not adhere toostrongly to the surface and can beeasily removed by a simple polishingoperation with a soft-wire brush.

Table 1 — Details of the nitrocarburizing (NC) and nitriding (N) experiments

Process Process parameters

Plasma NC Temperature, ºF (ºC) 1050 (566)Time, h 15%N2 50

Gas NC Temperature, ºF (ºC) 1050 (566)Nitriding Potential, atm-1/2 3Carburizing potential, Kc (W) 0.31Time, h 20

Gas N Temperature, ºF (ºC) 968 (520)Nitriding potential, atm-1/2 2Time, h 24

HEAT TREATING PROGRESS • MARCH/APRIL 2007 03

Compound zones formed in the gasnitrocarburized and nitrided sampleshad a very uneven thickness with aninconsistent compact portion and tree-like roots along the flakes of graphiteand between surface cracks and im-perfections (Fig. 3). Total penetrationof the internal structure by these rootsof the compound zone exceeded sev-eral times the thickness of the com-pact portion of the zone. Therefore,the specific volume of the near-sur-face layer became locally larger thanthat of the base matrix, which resultedin the areas of increased volume to ex-tend above the original surface. Fur-thermore, the cracking and plastic de-formation in this region added to theroughening effect.

In the as-plasma-treated sample,the compound zone was very uni-form and compact without any sig-nificant defects.

Industrial Case StudiesIn industrial applications, the neg-

ative effects of gas nitriding of graycast iron stamping and forming diesmay even be more visible than in thelaboratory because these types of thelarge castings very often have mi-crostructural defects, lack of homo-geneity, surface decarburization, andkish graphite flakes as shown in Figs.4 and 5[24]. It is possible that the“openings” near graphite flakes inthe surface of gray cast iron, such asthose in Fig. 5, allow ammonia topenetrate deeper into material.

DiscussionSurface defects of gray cast iron are

created during the machining oper-ations performed on the as-cast sur-face in the process of the finalshaping of the die geometry resultfrom the physical differences be-tween graphite and the mostlypearlitic matrix. Graphite flakesallow easy free machining of castiron by introducing discontinuitiesin the metal matrix[24, 25]. Therefore,during machining, chips are brokenrather than plastically deformed orsmeared over the surface by the cut-ting tool. As a result of the clear-cutmachining, surface voids and cracksare not filled with the metallic com-ponents of the matrix, but ratherwith graphite, or they stay open.Also, the interface between graphiteflakes and the matrix may not be ascoherent near the surface as in thecore.

During plasma nitriding of mate-rials having a porous surface, suchas cast iron, the hardened layers pro-duced are superior to layers pro-duced by gas nitriding. Plasma-pro-duced layers are very compact,which promotes a much smoothersurface. This is the result of the in-ability of the active species gener-ated in plasma above the cathode topenetrate small cavities and crevicespresent below the surface of the ma-terial being processed. These speciesare very likely N2+ ions, also consid-ered as precursors for forming any

Table 2 — Surface roughness data of samples after various treatments

Initial Gas Plasma

Temperature, ºF (ºC) 1050 (566) 968 (520) 1050 (566)

Ra, Measurement 1 3.31 203.64 121.67 35.67microin. 2 2.63 245.67 143.24 28.73

3 2.05 126.99 130.98 36.92

4 3.89 135.71 90.20 35.44

5 2.72 138.32 106.44 29.53

Pick Measurement 1 5 145 119 145count, 2 5 165 94 152picks/in.

3 0 114 102 157

4 0 127 114 170

5 0 119 107 170

Fig 3 — Photomicrographs of the near-sur-face area of gray cast iron samples after plasmanitrocarburizing at 1050ºF, or 566°C (a), gasnitrocarburizing at 1050ºF (b), and gas ni-triding at 968ºF, or 520°C (c). Etchant: 2%nital. Note a limited length of the compactportion of the compound zone on the left ofthe photomicrographs in the gas treated sam-ples.

a

b

c

Fig 4 — Photomicrograph of the near-sur-face area of gray cast iron sample taken from25,000-lb (11,350 kg) stamping die after in-dustrial gas nitrocarburizing. Gas nitrocar-burizing parameters: 1000°F (538°C) for 12h, nitriding potential and carburizing poten-tials not determined. Etchant: 2% nital. Notesevere decarburization of the near-surfacelayer with a network of kish graphite flakesand a deep penetration of the compound zone(white layer) along the flakes.

0.005 in.

0.005 in.

0.005 in.

0.020 in.

04 HEAT TREATING PROGRESS • MARCH/APRIL 2007

other ions in plasma nitriding[3].Very complex reactions betweenthose ions result in the formation ofactive nitrogen (N) atoms, whichhave a lifetime greater than 1.0second, and are the primary speciesresponsible for plasma nitriding[6].Although the mean free path of theN atoms along a straight line mightbe comparatively long dependingon the pressure used, their collisionswith the solid workpiece deactivatethem before they can enter the con-fined spaces below the cathode sur-face and, therefore, their range isvery limited. Additionally, productsof the reactive sputtering partiallycome back to the cathode surfaceand fill the cavities and micropores,minimizing penetration of the ni-triding species by simple mechan-ical masking.

In contrast to plasma nitriding,ammonia molecules in gas nitridingenter all of the confined spaces atfirst and then generate active ni-trogen atoms during collisions withthe solid (walls of the cavities).Therefore, they have a longer lastingeffect, and by penetrating deeply,produce very significant roots of thecompound zone below the surface(Fig. 6). Those processes increasespecific volume of the near-surfacelayer, and by causing local plasticdeformation of the material, lead to

Fig 5 — Surface of gray cast iron sampletaken from 25,000-lb (11,350 kg) stampingdie after industrial gas nitrocarburizing. Gasnitrocarburizing parameters: 1000°F (or538°C) for 12 h, nitriding potential and car-burizing potentials not determined. Note thelong groove in the surface around the graphiteflake on the right side of the picture.

Fig 6 — Schematic representation of nitriding of gray cast iron inside a near-graphite cavityby gas nitriding (left) and plasma nitriding (right). Active nitrogen atoms are marked as blackballs, N2+ ions are marked as a plus signs, and neutral nitrogen molecules are marked “O.”

1/2 island ad

HEAT TREATING PROGRESS • MARCH/APRIL 2007 05

local bulging of the surface, thus increasing its rough-ness.

ConclusionBased on the evaluation of experimental data, simple,

logical consequences of the differences between plasma-and gas-nitriding mechanisms demonstrate that there arelimitations to what degree results of these two methodsare identical. In processing products made of gray castirons, plasma nitriding does not penetrate deep into thematerial and, therefore, formation of the nitrided layer islimited to only the surface of the product. Therefore,plasma nitriding has no nitriding mechanism-limited ap-plication characteristics in processing of any cast or porousmaterials. On the other hand, gas nitriding, because of itsability to penetrate deeply into those materials,cannot be used easily in such applications.

Acknowledgement: The authors thank Henry Zmijewski atU.S. Steel Automotive Center, Troy, Mich., for his assistance withoptical and SEM metallography.

For more information: Edward A. Rolin’ski, Sc. D., is Vice President Technology, Advanced Heat Treat Corp. 1625 Rose St., Monroe, MI 48162; tel: 734-243-0063; fax: 734-243-4066; E-mail: doctorglow@ion-nitriding.com; Internet: www.ahtweb.com.

References1. H. Knüppel, et. al., Nitrieren von Stahl in der Glimment-ladung, Stahl und Eisen, Vol. 78, No 26, p 1871-1879, 1958.

2. H. Hornberg, Ein Verfahren zum Nitreiren von Stahlober-flächen mit einer Glimmentladung, Härterie Technische Mit-teilungen, Vol. 17, p 62-87, 19623. U. Seybolt. Some observation on the Metallurgy of Ion Ni-triding, Transactions of The Metallurgical Society of AIME, Vol.245, April, p 769-778, 1969.4. K. Keller, Schichtaufbau glimmnitrierter Eisenwerkstoffe,Härteie Technische Mitteilungen, Vol. 26, p 120, 1971.5. M. Hudis, Study of ion-nitriding, J. Appl. Sci., Vol. 44, No. 4,April, p 1489-1496, 1973.6. G. G. Tibbetts. Role of nitrogen atoms in “ion-nitriding, J. Appl.Phys., Vol. 45, No. 11, Nov., p 5072-5073, 1974.7. T. Karpinski, E. Rolin’ski, Mechanismus des Ionitrierens, Proc.8th Natl. Conf. on Heat Treatment, Ed. CSVTS Bratislava, Slo-vakia, p 27-35 1978.8. Marciniak, T. Karpin’ski, Comparative Studies on Energy Con-sumption in Installations for Ion and Gas Nitriding, Ind. Htg.,4, p 42-44, 1980.9. T. Lampe, et. al., Compound Layer Formation during PlasmaNitriding and Plasma Nitrocarburizing, Surf. Engrg., Vol. 9, No.1,p 69-76, 1993.10. H. Michel, et. al., Progress in the analysis of the mechanismsof nitriding, Surf. & Coat. Tech., Vol. 72, p 103-111, 1995.11. J. Conybear and B. Edenhofer, Progress in the control of Plas-manitriding and carburizing for better layer consistency andreproducibility, Proc. 6th Intl. Conf. on Ht. Trt. of Matls. Sept.Chicago, p 381-394, 1988. 12. A. Sokolowska, et. al., Nitrogen transport mechanisms inlow temperature ion nitriding, Surf. & Coat. Tech., 142-144, p1040-1045, 2001.13. E. Lehrer, Über das Eisen-Wasserstoff-Gleichgewicht, Z. Elek-trochem. Vol. 36, No 6, 383-392, 1930.14. H. Grabke, Reactionen von Ammoniak, Stickstoff andWasserstoff an der Oberflache von Eisen, Ber. Bunsengessell.Phys. Chem., Vol. 72, p 533-548, 1968.15. E. Rolin’ski and G. Sharp, When and Why Ion Nitriding/Ni-trocarburizing Makes Good Sense, Ind. Htg., Aug. p 67-72, 2005.16. T. Brinke, et. al., Plasma Assisted Surface Treatment, Ed.Verlag Moderne Industrie, 2006.17. T. Bell, et. al., Controlled nitriding in ammonia-hydrogenmixtures, Ht. Trt. p 51-57, 1973.18. S. Böhmer, et. al., Überwachung von Verfahren zur Erzeu-gung nitridhaltiger Schichten, Neue Hütte, Vol. 10, p 384-390,1979.19. S. Böhmer, et. al., Oxygen Probes for Controlling Nitridingand Nitrocarburizing Atmospheres, Surf. Engrg., Vol. 10, No.2,p 129, 1994.20. H. Klümper-Westkamp, Sensorentechnik für das Aufkohlenund Nitrocarburieren von morgen, GASWÄRME Intl., Vol. 55,No. 4, p 272, 2006.21. K. M. Winter, Nitrocarburieren mit unabhänging geregeltemNitrier- und Carburierenpotenzial, GASWÄRME Intl., Vol. 55,No. 4, p 277, 2006.22. Index for Stamping Dies Cast Materials in NAAMSStamping, Global Standard Components, Auto/Steel Partner-ship, 199723. E. Rolin’ski, et. al., Plasma Nitriding Large Stamping Dies,Ht. Trt. Prog., Sept/Oct., Vol. 6, No. 5, p 19-23, 2006. 24. Metals Handbook, Properties and Selections: Irons, Steelsand High Performance Alloys, Vol. 1 Tenth Ed., ASM Intl., 1990.25. Metals Handbook, Machining, Vol. 16 Ninth Ed. ASM Intl.,1989.

pÉÉ=óçì=áå=abqolfq2007 ASM Heat Treating Society

Conference and ExpositionSept 17 - 19

For more information, visit the web at www.asminternational.org/events

06 HEAT TREATING PROGRESS • MARCH/APRIL 2007